Refactoring the jupyter notebook

[FanwangM]

The current https://github.com/FanwangM/Selector/blob/main/book/content/starting.ipynb is somewhat hard to follow and we need to clean it up.

• [x] Put synthetic data with and without cluster labels into two notebooks;
• [x] Add a practical example to show how one can select compounds given a chemical library.
• [x] Address https://github.com/theochem/Selector/issues/208.

[FanwangM]

This is fixed in #186.